Ross Williams, then as now the chairman of Pontypool and District Schools rugby club, had stepped in to deliver some light coaching of the juniors when Billy, on a familiar bull-like charge, banjaxed him right in the midriff. Dawson Jones, the youngster’s coach, can barely conceal his own mirth as he recalls the sound of Williams’ helpless groaning.

Here in Pontypool, folk know all about the Vunipolas’ awesome violence with a rugby ball. Billy and his elder brother, Mako, spent most of their childhoods in this place – such that Mako continues to speak with a slight Gwent lilt – but nobody ever remembers them looking like little boys. “They were enormous,” says Jones, or ‘Uncle Daws’ as he still is to Billy. “They would have saucepans of potatoes, not even chopped. The food they used to put away was extraordinary.”

Jones is the younger Vunipola’s guest of honour at Twickenham on Saturday, and amid the tension of the occasion he could be forgiven for reflecting how it came to this. How on earth did the 128th collision between England and Wales, the most significant in the Six Nations since 2013, come to be be decided by the battle of two No 8s whom he once ordered around for East Wales Under-11s?

When he looks at a memento of the team’s summer tour of Italy in 2002, he is struck by the names of three boys clustered in the top right of the photograph: Billy Vunipola, Mako Vunipola and Taulupe ‘Toby’ Faletau. In the decade since, Mako and Faletau, equally exceptional during this Six Nations, have both had their baptism as British and Irish Lions, while Billy, at just 23, has morphed into the most destructive No 8 in the northern hemisphere. The brothers were prised away by England while Faletau, a star in the Welsh system since he was 10, has stayed true to the red jersey. But ‘Uncle Daws’, their kindly mentor, will always regard their story as the property of Pontypool.

“I found this letter that Billy wrote to me,” says Jones, in his living room on the outskirts of town. “I did promise that I’d never breathe a word about it to the press. But he wrote out a thank-you about playing for Pontypool Schools, and his final statement was: ‘I promise, Uncle Daws, I’m never playing for England.’”

Once Billy, born Viliami, moved with his family and later won a scholarship to Harrow, he was perceived to have been lost to Wales. But clandestine efforts have been made to force him back. On the evening that Wales trounced England 30-3 in Cardiff three years ago, it seemed fleetingly as if he could be persuaded.

Fe’ao, his father and a former Tongan international, met Jones for a drink at the Sandringham Hotel and whispered: “Billy can still play for Wales, you understand?” Jones replied: “I know. What more do you want me to do? I’ve tried my hardest.” Either Billy or Mako, he considers ruefully, could have been brought back as exiles. They had lived in Pontypool for over three years and met the eligibility requirements. “A few months later, though, Billy was picked him to play for the Saxons. It was Wales’ loss and England’s gain.”

To understand the true depth of gratitude that the Vunipola brothers and Faletau – who, contrary to urban myth, is not a cousin of theirs, but more of an adopted sibling – owe to their upbringing in Wales, it helps to skip back a generation. For when Fe’ao Vunipola arrived in south Wales, careworn from jet-lag, in the summer of 1998, he had nothing. Granted, he had his rugby heritage, as the son of Sione Vunipola, a policeman who coached both the 1972 and 1995 Tongan teams in New Zealand. His grandfather, too, belonged to Tonga’s original international team in 1924. But in any material sense, Fe’ao, who had travelled 12,500 miles in search of economic opportunity, was all but destitute.

Fortuitously, he soon made the acquaintance of Terry Gordon, Pontypool’s long-time kit man and known throughout local circles as ‘Tiger’. When Fe’ao eventually found lodging in a flat in Cwmbran, owned by the club’s then benefactor Jeff Taylor, Terry took it upon himself to offer charity. “There was no heating in the place, so Fe’ao had to go to bed with his overcoat and trousers on,” he remembers. “He had no food, either. So, I used to take Wagon Wheels and Jaffa Cakes over. He lived that way for quite some time, or at least until our fitness coach at Pontypool bought a three-piece suite for him.”

They were an odd couple, this avuncular character from the Valleys and the penniless Polynesian emigré. But the English rugby fraternity has much, ultimately, to thank them for. For without Terry’s intervention, Fe’ao would never have had the confidence to send for his wife, Iesinga, to join him with their three children. “He is my best mate in the world,” Terry says of Fe’ao, who is back in Nuku’alofa as interim president of the Tongan union. “He thinks the world of me and I think the world of him. He is a wonderful man.”

Jane, his riotously funny wife, takes up the story from the lounge of their terraced house in Machine Meadow, Pontnewynydd. For it fell to her, once Billy and Mako arrived as toddlers in late 2000, to try to ease their assimilation into the Welsh way of life. The challenge was not always straightforward. “The first night they came over, Billy and Mako went straight to the park. The children all had beanie hats, ready for the weather. One lady kept fiddling with Mako’s hat. So what did he do? Spit straight in her face.”

Others in Pontypool relate similar, albeit marginally less eye-watering, tales of cultural dislocation. According to Jones, Billy and Mako badgered him to drive them to Brecon at the first sign of snowflakes, whereupon they rolled around in the powder for so long that they spent the next three days dosing up on Lemsip. Even at the recent wedding of Faletau’s brother, Josh, the brothers did little to conceal their roots in the South Pacific sunshine.

“The ceremony was at half past 12, but most of the party didn’t turn up until quarter past two,” says Jane, suppressing a shudder at the memory. “Then the vicar looked down and Billy and Mako’s feet and said, ‘Erm, you do realise you’ve got flip-flops on?’ They had to send to send the groom all the way back to Cardiff to pick their shoes up.

"Naturally, he was late coming back. They had this big white limousine going round and round the valley. Then they asked Josh for the ring and he said, ‘I don’t know what I’ve done with it.’ So do you know what they got married with, in the end? A Fabergé egg.”

For all the chaos, the bond between the Gordons and the Vunipolas remains palpable. Iesinga works today in High Wycombe as a minister in High Wycombe but owes much of her success, just like Fe’ao’s, to the benevolent Welsh couple who took them in. “Fe’ao played for Pontypool and worked as a quantity surveyor in Cardiff for short periods, but he could never get a visa,” Jane explains. “The kids were going to have to go back to Tonga.”

The family’s devout Methodist faith, it turned out, provided an answer. Each Sunday, Billy and Mako would play touch rugby with Toby, their fellow Tongan, on the Pontypool village green before Mass. If Fe’ao could not secure a work permit, Jane reasoned, then she would enrol Iesinga for the theological college in Cardiff. The college accepted her, and the rest of the Vunipola’s lives was set.

Billy and his brethren never forget these roots. The junior Vunipola is thriving under the stewardship of Eddie Jones with England and yet, even amid the euphoria of a man-of-a-match display in the Calcutta Cup last month, his first act in the Murrayfield dressing room was to send a text to Dawson Jones in Pontypool. Jones proudly shows off the message. It simply reads: “I seem to have good luck with coaches called Jones.”

Equally, Faletau, now dating Gareth Bale’s sister-in-law, is prospering with Warren Gatland, having delivered a tackling masterclass in the win over France to set up this titanic confrontation with England. A theory persists that he could also have escaped Wales’ clutches when he joined Filton College in Bristol, until a coach at Newport-Coach Dragons spotted his talent and took him back across the border. Jones, however, scoffs at the suggestion. “There’s a picture of Toby as an 11 year-old wearing his East Wales Schools jersey, still hanging on the wall at Pontnewynydd Primary School,” he says. “We all knew he was brilliant then.”

On their present trajectories, the Vunipolas and Faletau could easily end their careers with a ton of Test caps apiece. All three would be considered frontrunners to make the Lions tour in New Zealand next summer. But for the Gordons, it is sufficient merely to know that this Tongan triumvirate still think about them from time to time. “We were giving them a chance in life, I suppose,” says Jane, mistily. Refined in England they might have been, but they were all made here in Pontypool.